In sulfuric acid alkylation, an olefin is reacted with isobutane in excess in the presence of sulfuric acid catalyst. Although there are several variations of the alkylation process, for example, the Stratford effluent refrigeration system and the Kellogg autorefrigeration system, the effluent from the reaction system typically includes separated acid (which may be recycled to the reactor), a liquid hydrocarbon phase including alkylate product and a vaporous light hydrocarbon effluent. The latter may be directly from the reaction vessel as in the case of the Kellogg autorefrigeration system or from a suction trap-flash drum arrangement as in the Stratford effluent refrigeration system. The liquid hydrocarbon effluent is typically ultimately passed to a distillation zone from which a vaporous fraction is typically removed overhead, largely comprising excess isobutane and a liquid bottom fraction, particularly containing product alkylate, is also removed.
It has been proposed in the prior art to employ one or both of the vapor fractions from the reaction step and the distillation step, after compression, as a heat exchanging medium for the liquid hydrocarbon effluent, either with respect to an effluent flash system before the distillation step or of the distillation step itself. When these vaporous fractions are separately employed with separate compressors and circulation systems, with the different fractions being handled and recycled independently of one another, corrosion problems may not arise. However, when, as is most efficient, it is desired or attempted to use the two vaporous fractions from the reaction step and distillation as a common feed to a common compressor, before heat exchange, problems arise which have caused the development of the instant improvement.
With respect to the distillation of the liquid hydrocarbon effluent in the distillation tower or deisobutanizer, if this feed is not first neutralized, then, when the distillation tower is run at temperatures and pressures required to obtain the results necessary (even in a relatively low pressure tower), acidic components, present in the hydrocarbon phase effluent, when heated in the reboiler of the distillation or deisobutanizer tower, will decompose and foul the reboiler, preventing operation. In order, therefore, to provide practical operation of the distillation tower, the liquid hydrocarbon phase effluent necessarily must be neutralized (typically caustic and water wash) before reaching the distillation or DIB tower.
The result of the latter step is that the liquid hydrocarbon phase effluent, after the caustic treatment and water wash, will contain water or be wet. Inevitably, overhead isobutane vapors from the distillation tower will contain traces of water. When one considers that the reaction vaporous effluent, either from a suction trap-flash drum system or directly from an autorefrigeration reactor, is necessary dry and contains sulfur dioxide vapors, then it may be seen that the mixing of the distillation column wet overhead and the dry acidic reaction vaporous effluent will cause the formation in the mixture of highly corrosive weak acid. The use of such as a heat exchange medium in exchangers or reboilers will damage them and render them inoperative.
In the Stratford effluent refrigeration system, the suction trap-flash drum vapors are necessarily dry and acidic, because the hydrocarbons have been in contact with excess strong acid in the reaction phase and were separated in the acid settler without any water contamination or neutralization. Likewise, in the autorefrigeration system of Kellogg, the hydrocarbons, which have been in contact with excess strong acid in the reaction phase, are separated in their reaction vessel without any water contamination or neutralization. Should the overhead isobutane vapors from the distillation tower containing traces of water being mixed with the dry, acidic, sulfur dioxide containing vapors from the reaction side or system, the mixed vapors become very corrosive (weakly acidic). Thus, there is not only the likelihood or definite possibility of corrosion at the compressor itself, but also definite, unavoidable, necessary corrosion in the reboiler of the deisobutanizer tower or heat exchanger or condenser of an effluent flash vaporization system. Even where bauxite neutralization is employed, there remains a threat of water contamination of the DIB overhead vapors.
The present system makes it feasible to perform the necessary neutralization on the liquid hydrocarbon phase effluent before it is distilled in the deisobutanizer tower, but yet also permits the mixing, in a common compressor, of the normally incompatible reaction phase vapor effluent and distillation tower vapors overhead to provide a compressed heat exchanging medium which is not in any way corrosive, thereby protecting both the compressor and the reboiler of the distillation tower or whatever heat exchanger at which the combined, compressed light hydrocarbons may be employed.